Since powder coating is carried out using powder coating materials which contain no solvent, unlike a coating method using solvent type coating materials, the powder coating is free from environmental issues which could be caused by solvents or a risk of occurrence of disasters and further over-sprayed coating materials can be recovered and made usable again, and therefore the powder coating can suppress the coating material loss to very low and has thus been employed in various coating fields, particularly in metal coating. Further, in terms of usability of conventional coating apparatus for water-based coating materials, coating methods using powder type water-based coating materials obtained by dispersing powder coating materials in water have been proposed and they are coating methods suitable for decreasing VOC (volatile organic compounds).
On the other hand, matte finish has often been done to render high grade feeling along with preference of high level for domestic electric appliances and interior panels of constructions. As powder coating materials to be used for their coating have widely been employed thermosetting powder coating materials containing mainly polyester resins, acrylic resins, and epoxy resins since they are excellent in coating film performance.
Conventionally, formation of a matte coating film is carried out by adding an inorganic matting agent such as silica and alumina to powder coating materials, however, a small amount of a matting agent is insufficient and if a large quantity of a matting agent is employed, it leads to a problem in wear of production apparatus for powder coating materials and deterioration of physical properties of coating films after coating.
For example, Japanese Patent Application Laid-Open (JP-A) No. 10-306237 proposes matting powder coating materials which are powder coating materials mixed with inorganic matting agents and obtained by depositing mica powder on the surfaces of powder coating material particles containing barium sulfate. However, the brilliant luster of the mica powder remains and uneven luster may sometimes occur. Further, there occurs a problem that undercoat luster (80° luster) remains.
Further, JP-A No. 2003-55614 proposes matting thermosetting powder coating materials obtained by spray drying organic solvent solutions containing thermosetting resins, curing agents and matting agents, and having an average particle diameter of 5 to 10 μm, however, in terms of reduction of VOC, it is needed to recover evaporated solvents and it makes complication of facilities.
Furthermore, JP-A No. 2003-192992 proposes matting agents for coating materials containing one or more kinds of inorganic fine powders selected from fine silica powder, silica-alumina, and fine alumina powder which are surface-treated with waxes containing mainly higher fatty acid esters, and energy curable compositions for coating material containing the foregoing matting agents for coating materials.
However, the above-mentioned matting coating materials containing the matting agents still have many problems in terms of workability and apparatus cost in production of the powder coating materials, relevant to environmental issues and machine wear, although improvement of the properties as powder coating materials is considerably expected.
For example, in the case of producing the powder coating materials in a dry manner, resins for forming coating films, curing agents, matting agents, stabilizers and so forth are mixed and subjected to particle size adjustment by pulverization and particle control to obtain powder coating materials. In this case, if hard inorganic components such as silica and alumina are mixed with organic components such as resins, just like sand erasers as a stationery, the hard inorganic components mixed with the resins act like abrasives, possibly cause a wear problem of a mixer, a pulverizer, a particle controller, a feeder equipped with them, a transportation tube, a storage tank, and so forth, and thus require countermeasures to wear and repair of apparatuses. Further, powder soaring tends to be easily caused due to collision of powder coating materials themselves. It is also difficult to remove wear debris of the apparatuses mixed in coating material products. In a powder coating step, it is said that the air speed near a diffuser part and a nozzle is a subsonic speed, and the transfer speed of the coating materials in a coating material hose sometimes reach ten and several meters per second and it also causes a wear problem of the hose.
On the other hand, in the case mixing is carried out in a water-based or solvent-based system, after mixing, dewatering or desolvation is carried out and successively spray drying or fluidization drying is carried out to obtain powder coating materials. In the case of carrying out the mixing in a water-based or solvent-based system, even if the hard inorganic components are mixed, it is made possible to employ a production method which does not include a step of melting and kneading and a step of mechanical pulverization, and, in this case, there is no need to consider machine wear in the step of melting and kneading or the step of mechanical pulverization, however, there occurs a problem of wear of facilities in other steps, e.g. in particle control step and a drying step. Further, in the case of a solvent-based system, there are raised additionally problems such as cost up because of VOC countermeasure and explosion prevention designs of apparatuses.
Although having a matte effect, inorganic powders such as silica which a Mohs hardness is higher than 4, lack in the surface smoothness in particular in semi-matte finish and have a problem in the appearance beauty supposedly because the particles have many edges.
Further, in the case of inorganic substances containing silica, for example, in a mixing step, a pulverizing or particle control step, or a drying step, the powder dust of substances may be formed and risk of silicosis is considered to be possible, so that particular care has to be paid for the work environments.
Further, in a coating step, since excess powder coating materials are recovered and reused, in the case of using powder coating material containing hard inorganic components, there are problems of powder soaring due to collision between powder coating material particles and wear of coating apparatus and recovery apparatus.
The present invention aims to solve the above-mentioned problems in conventional techniques, provide a filler for powder coating materials which hardly cause a problem of machine wear at the time of production and can form a coating film excellent in matte properties, finishing properties (appearance), processability, and impact resistance, and provide a powder coating composition containing the filler.